1. Field of the Invention
The present invention related generally to DC-DC voltage converters and, in particular, to synchronous rectifying DC voltage converters.
2. Description of the Prior Art
Electronic devices are generally supplied with power from a power supply unit that converts AC line power to a DC voltage. However, various components in the electronic devices may require different voltages than is supplied by the power supply. In addition, some of these components require a highly regulated voltage to operate properly. DC to DC converters are used to convert the output of the power supply unit to a different, regulated voltage.
Electronic devices which may use DC to DC converters are computers, network cards, DSL (Digital Subscriber Line) cards, and the like. The DC to DC converters may be required to produce a 5 volt regulated output, or in some cases a 3 volt regulated output, 12 volt regulated output or 15 volt regulated output, for example. In a computer, the power supply unit may produce one voltage for supply to components such as disk drives while another lower voltage is required for the memory chips or processor. It is important in such applications that this lower voltage level be maintained as precisely as possibly since the logic circuitry depends upon voltage levels for accuracy. This is true even when current demands are being placed on the main power supply for instance during start-up when the disk drives are operating and drawing more power than usual.
One type of DC-DC voltage converter is known as a synchronous rectifier. The term synchronous rectifier refers to the active element, here a FET operating synchronously with a conducting state of a diode. Synchronous rectifiers are low loss devices. An example of a synchronous rectifier is shown in FIG. 1 in the configuration of a self-driven synchronous rectified forward converter. The device is self-driven by virtue of deriving the driving voltage from the secondary. A cross-connected configuration provides the self-driven aspect, as opposed to use of an external control circuit which would not be self-driven.
In FIG. 1, a DC voltage is applied at an input 10. Also at the input side of the DC-DC power converter is a transistor 12 which serves as a power switch. An inductor 14, a capacitor 16, and a pair of diodes 18 and 20 complete a resonant primary snubber circuit. This snubber circuit is connected to a primary side 22 of an isolation and step-down transformer 24. The secondary 26 of the transformer 24 includes two synchronous rectifiers 28 and 30. The rectified signal from the secondary 26 is filtered by an averaging filter made up of an inductor 32 and a capacitor 34. The node between the inductor 32 and the capacitor 34 serves as an output node to which is connected a load of the DC-DC power converter stage.
A non-synchronous construction of the circuit is also possible by replacement of the transistors or rectifiers 28 and 30 with Schottky diodes.
A problem with the foregoing circuit is that the secondary voltage from the transformer 24 appears directly across the gate-to-source of both of the rectifier elements 28 and 30. Some synchronous rectifier field effect transistors have a maximum gate voltage rating as low as 12 volts. This precludes their use in this type of circuit for all but the lowest output voltages.
Thus, in a self-driven synchronous rectifier, peak gate voltages can easily exceed the gate-source breakdown voltages of the synchronous FETs, particularly at 5 volt outputs or greater. This leads to destruction of the FET and failure of the device. Therefore, it has been required to limit the self-driven synchronous to an output voltage of 5 volts or lower.
One possible way to limit the gate voltages of the FETs is to connect Zener diodes across each of the gate-to-source leads. The Zener diodes limit the voltages but do so by conducting current when the limit voltage is met, resulting in lost energy through the Zener. Thus, this is a lossy circuit. It also results in the nominal output voltage being limited to the Zener voltage.
An alternative is to use integrated circuits to control the FETs, so that the circuit is no longer self-driven. This permits more power to be supplied by the circuit, however, the use of such IC is costly.